Login System (with database file)

[Miffiq]

Alright, so I need a program that will read to see if a database file exists, and if it doesn't, asks the user to create a username and password, then put them in the database file. Then regardless of the result of the file check, ask to login. When the user logs in, I want it to be the username and password that is in the database file.

ALSO:
I want to be able to have multiple users.
I would PREFER to have one database file, and not two, as this complicates things.
Say there are 5 users. I want user 4 to be able to edit JUST their username and password from the database.
I want to have a guest mode.
I want to have ONE admin.

I know that is a lot, but I have a code that will do most of it, there are just a few problems.

#include <cstdlib>
#include <vector>
#include <iostream>
#include <fstream>
#include <string>
#include <stdio.h>
#include "functions.h"


using namespace std;

const int SIZE = 100;	// Always use MORE than required
string Usernames[SIZE]; // From the database
string Password[SIZE];	// From the database

int GetNameIndex(string query, int size)
{
	for (int i=0; i<size; i++) //Prevents the program from reading garbage
	{
		if (query == Usernames[i]) return i; //Return the index
	}
	return -1; //Error code
}
bool PasswordMatches(int index, string passwd)
{
	return (passwd == Password[index]);
}



int main(int argc, char** argv) {   
    initUserCheck(); // Checks to make sure that a user is registered, if not creates a new one.
    
    //Starts the Login Process.
    //Read the database:
        
        const char *appdata = std::getenv ( "APPDATA" );
        std::string path = appdata;
        path += "\\myprogram\\files\\database.dtbf";
        std::ifstream fin ( path.c_str() );
        
	int i=0;

	while (!fin.eof()){
		fin >> Usernames[i] >> Password[i];
		i++; //This represents the number of lines we could extract from the database
	}

        //Begin VISUAL login
        RetryLogin:
	string usrname, passwd;
	printf("Username: ");
	cin >> usrname;

	int index = GetNameIndex(usrname, i); //Send the database size to this function.

	printf("Password: ");
	cin >> passwd;
                        
	if (!PasswordMatches(index, passwd)){ // If Password+Username doesn't match.
	
            printf("Correct Username, incorrect Password.  Please try again.  \n");
            system("PAUSE");
            goto RetryLogin;
	}
    
    fin.close();
    printf("You have successfully logged in.\n\n");

    // Rest of the Program... 

Probems with this code:
If a user enters a wrong username, the program freezes, and then becomes unresponsive (in English, it crashes). There is no guest, or admin mode.

Last thing:
lets say that the file, filename has this:

sillyguy
password
mike
legos
sarah
ilikepie

How does mike, edit JUST their username and password, without messing with everyone elses?

Thanks!!

If you don't get anything about this, I will get back to you ASAP.

[BlackSheep]

Files are treated as streams, so you can't go into the middle of a file and edit specific parts directly. You'll need to create a second file, read the lines out of the first one, possibly change them, write them to the second file, delete the first file, rename the second file.
Isn't C++ wonderful?

[modoran]

Use sqlite library, it was designed fot this kind of purpose:
http://www.sqlite.org/

[Miffiq]

Files are treated as streams, so you can't go into the middle of a file and edit specific parts directly. You'll need to create a second file, read the lines out of the first one, possibly change them, write them to the second file, delete the first file, rename the second file.
Isn't C++ wonderful?

How should I go about doing that, should I completely pitch the code I currently have? Can you show me some source code?

[Miffiq]

Figured it out myself. Just sat down and started thinking... Anything wrong with this?? It meets all of my requirements except for the one file thing... but whatever.

#include <cstdlib>
#include <vector>
#include <iostream>
#include <fstream>
#include <string>
#include <stdio.h>
#include "functions.h"


using namespace std;

int main(){    
    initUserCheck(); // Checks to make sure that a user is registered, if not creates a new one.

    Login:
    string usrname;
    string psword;
    string rpsword;
    string path;
    string ext = ".DTBF";
    int adminLevel = 0;
    
    printf("Username: ");
    cin >> usrname;
    printf("Password: ");
    cin >> psword;
    
    const char *appdata = std::getenv ( "APPDATA" );
    path = appdata;
    path += "\\myprogram\\files\\users\\" + usrname + ext;
    
    ifstream readUser(path.c_str());
    readUser >> rpsword;
        
    if(!readUser){
        
        if(usrname == "Guest" or "guest"){
        
        printf("You are a guest.  \n");
        goto MainProgram;
        
    }
        printf("Incorrect Username or Password.  \n");
        system("PAUSE");
        printf("\n");
        goto Login;
        
    }
    
    if(psword != rpsword){
        
        printf("Incorrect Username or Password.  \n");
        printf("Please press any key, and then retry the login.\n");
        cin.get();
        goto Login;
        
    }
    
    if (usrname == "admin" or usrname == "Admin"){
        
        adminLevel = 1;
        
    }

MainProgram:
//MainCode 

If you have any things to say about improving this login system code, then feel free. Thanks guys!!